Personally, having been up through Gruul, I remain a big fan of Jeklik in Zul'Gurub-- it's got just the right mix of old fashioned tanking, AoE, and (eventually) all-out chaos that a great boss battle should have. People are saying that C'thun is a classic, but like most players, I've never had the chance to see him. In terms of five-man bosses, the quality on those really jumped up in Outland. I'd say Heroic Capacitus is fun, with the positive/negative mechanic borrowed from Thaddius. And Sepethrea's little Tron room in the Mechanar is a good time, too.
But I'm probably forgetting quite a few good ones. What is the best boss fight in the game?